Many processes have been proposed for separating metals, one from another. One commercially significant method comprises selective extraction of one metal from another from an aqueous solution of the pair using a solvent-extractant in which the first metal valve is preferentially soluble, leaving the second metal behind, in the aqueous solution. Among the most difficult to separate metal pairs are copper from iron, nickel from iron, nickel from cobalt, copper from cobalt, copper from nickel, copper from zinc, zinc from nickel, nickel from zinc, zinc from iron, and cobalt from zinc, and the like.
In Swanson, U.S. Pat. No. 3,224,873, a family of aliphatic .alpha.-hydroxy oximes is disclosed to be useful in solvent-extractant processes for recovering copper from aqueous solutions derived from acid leaching of ores. In Swanson, U.S. Pat. No. 3.428,449, a family of 2-hydroxy benzophenone oximes is used to recover copper in the presence of iron, and it is disclosed that a small amount of aliphatic .alpha.-hydroxy oximes enhances the rate of extraction of copper. Hazen and Coltrinari, U.S. Pat. No. 3,872,209, have found that the rate of extraction of copper is enhanced by using an organophosphoric acid additive in the solvent-extractant containing a 2-hydroxy benzophenone oxime. This permits the use of shorter contact times and smaller mixer-settlers. Morin and Peterson, U.S. Pat. No. 3,878,286, have discovered that the rate of extraction of copper by solutions of aliphatic .alpha.-hydroxy oximes and 2-hydroxybenzophenone oximes is enhanced by the addition of small amounts of sulfo compound, such as a dialkyl sulfosuccinate, an alkyl naphthalene sulfonic acid or a long chain monoalkyl sulfate. Moreover, the use of the sulfo additives were reported to increase the selectivity of the oxime reagents for copper in the presence of iron. With particular reference to the alkyl naphthalene sulfonics, Morin and Peterson state that the proportion of sulfo compound can go from 0.05 to 13 parts by weight per 100 parts of hydroxy oxime component. Higher amounts are said to lead to emulsions, which are undesirable in a mixing-settling operation, for obvious reasons. In any event, the Morin and Peterson disclosure clearly teaches that maximum mole % of alkylnaphthalene sulfonic acid in the total combination of acid and .dbd.-hydroxy oxime, assuming that dinonylnaphthalene sulfonic acid and 5,8-diethyl-7-hydroxydodecan-6-one oxime are used, is 7,1 mole %. The disclosures of the above-mentioned patents are incorporated herein by reference.
It has now been discovered that if an aliphatic .alpha.-hydroxy oxime is used in combination with a high molecular weight alkylaromatic sulfonic acid and if the ratio of sulfonic acid to oxime taught to be the maximum by Morin and Peterson is substantially exceeded, e.g., by more than 40%, in the extraction of acidic solutions of copper, nickel, iron, cobalt and zinc, individually or in combination, the selectivity of the extraction and efficiency are remarkably increased. No emulsification problems are encounter, although, if equivalent mixing is used, longer separation times are required, but extraction equilibrium is achieved rapidly, and therefore long and intensive mixing is not required. Tests have shown that the equilibrium extraction efficiency or aromatic hydroxy oximes, such as 2-hydroxy-5-nonyl-benzophenone oxime, is not improved by the use of the additive in the amounts specified herein, and in fact an antagonistic effect is observed. Moreover, if the mole % of sulfonic acid is lowered, with respect to the total of sulfonic acid and .alpha.-hydroxy oxime, into the range used by Morin and Peterson, the loading of extracted metal values decreases to an undesirable level, and maximum selectivity cannot be achieved.